1. Field of the Invention
The present invention relates to a sampling frequency converter for converting input data sampled at a sampling frequency of M into data sampled at a sampling frequency of N.
2. Related Background Art
In order to convert input data sampled at a sampling frequency of M=F * m.sub.1 * m.sub.2, . . . , * m.sub.k (m.sub.1, m.sub.2, . . . , m.sub.k are positive integers) into data sampled at a sampling frequency of N=F * n.sub.1 * n.sub.2, . . . , * n.sub.k (n.sub.1, n.sub.2, . . . , n.sub.k are positive integers), a conventional sampling frequency converter such as shown in FIG. 4 has over-sampling filters Oi for over-sampling the input data by an n.sub.i -fold and down-sampling filters Di for down-sampling by an m.sub.i -fold.
However, at each stage of pairs of the cascaded over-sampling filters Oi and down-sampling filters Di, the value n.sub.i /m.sub.i may become n.sub.i /m.sub.i &lt;1. In this case, the input data is first over-sampled by the n.sub.i -hold and then down-sampled by the m.sub.i -hold. Therefore, the sampling frequency lowers, and the margin between the upper limit of the signal frequency band and the lower limit of the folded component becomes small. The over-sampling filter at the next stage is therefore required to have a sharp cut-off characteristic, increasing the number of filter taps and the size of hardware.
More specifically, as shown in FIG. 5, consider the conversion of input data sampled at a sampling frequency of 48 kHz into data sampled at a sampling frequency of 44.1 kHz. In this case, M=48 kHz=300 Hz * 5 * 8 * 4, and N=44.1 kHz=300 Hz * 7 * 7 * 3. The number i of stages of the cascaded over-sampling filters and down-sampling filters is three, and F=300, m.sub.1 =5, m.sub.2 =8, m.sub.3 =4, n.sub.1 =7, n.sub.2 =7, and n.sub.3 =3. The sampling frequency converter is therefore constructed, as shown in FIG. 5, of over-sampling filters 11, 13, and 15, and down-sampling filters 12, 14, and 16.
In the circuit arrangement shown in FIG. 5, at the input side of the second stage over-sampling filter 13, the sampling frequency is 48 kHz * 7/5=67.2 kHz. If the signal frequency bandwidth is 20 kHz, the margin to the lower limit of the folded component is 27.2 (=47.2 -20) kHz as shown in FIG. 6A. At the input side of the third stage over-sampling filter 15, the sampling frequency is 58.8 kHz (=67.2 kHz * 7/8) and the margin to the lower limit of the folded component is 18.8 kHz. As a result, the number of taps of the third stage over-sampling filter 15 becomes greater than the second stage over-sampling filter 13, increasing the size of hardware.